This project is focused on understanding clathrin-independent forms of endocytosis (CIE). Endocytosis that occurs without clathrin coats occurs in all cells but is poorly understood. We are interested in studying the cargo proteins that enter cells by this mechanism, their intracellular itinerary once they have been internalized and whether they contain amino acid sequences that allow for specialized sorting within cells. We have been identifying new cargo proteins and found that a subset of these proteins take alternative traffic routes once they have entered cells. The major histocompatibility complex Class I protein (MHCI), is a prototypical clathrin-indepenent cargo protein and after internalization it reaches endosomes that contain cargo proteins such as the transferrin receptor that enter via clathrin-depenent endocytosis. From there, MHCI travels either to late endosomes and lysosomes where it is degraded or on to recycling tubules that return MHCI back to the cell surface. CD44, CD98, and CD147, however, show an altered itinerary in many cells where they traffic directly into the recycling tubules and avoid trafficking to lysosomes. Consistent with this altered itinerary, CD44, CD98 and CD147 are long-lived proteins and are not degraded like MHCI, which is routed to lysosomes. We have determined that human Jurkat T cells exhibit CIE and also sort cargo in a manner similar to that observed in HeLa cells (Johnson (Theisen) et al 2017). CD98 remains in the periphery while MHCI traffics to endosomes associated with the early endosomal antigen 1 (EEA1) and then on to lysosomes for degradation. As observed in other cell types, Arf6 and Rab22 are associated with CIE in T cells and their activities modulate endosomal traffic. Disruption of the GTPase cycle of Arf6 or Rab22 leads to impairment in T cell conjugation with Raji antigen-presenting cells. We are interested in the role that ubiquitin modification of cargo proteins plays in regulating the sorting of cargo and routing of cargo to lysosomes for degradation. We previously showed that over expression of the MARCH8 E3 ubiquitin ligase leads to ubiquitination and degradation of CD98, normally a long-lived protein (Eyster et al 2011). We also published that expression of a de-ubiquitinase (DUB) or ubiquitin-specific protease (USP) can counteract the effect of expression of MARCH8. This DUB is called TRE17/USP6 and was previously shown to be unregulated in ABC bone cancer. Expression of TRE17 specifically de-ubiquitinates CD98 and returns its trafficking to the normal pathway (i.e. avoidance of lysosomes) (Funakoshi et al, 2014). However TRE17 did not rescue the effect of MARCH8 on degradation of the transferrin receptor. We are now analyzing the results of an siRNA screen of human de-ubiquitinases (DUBs), knocking down the 100 DUBs in the human genome. We hope to identify those DUBs that, in addition to TRE17 (USP6), are responsible for protecting CD98 and CD147 from lysosomal degradation. We obtained several hits and are currently validating these hit and following up on them with further studies.